NK cells can kill tumor cells without prior sensitization, suggesting that they play important role in tumor surveillance. They also play an important role in innate immunity and autoimmunity. Enhanced susceptibility to killing by NK cells was first related to decreased expression of target cell MHC class I molecules, a finding that led to the "missing-self hypothesis. If MHC class I expression is downregulated, as in tumors or viral infection, NK cells are released from the inhibitory influence of MHC class I on the target cells resulting in target cell lysis. This tolerance mechanism is explained by MHC class I specific NK cell inhibitory receptors containing immunoreceptor tyrosine-based inhibitory motifs (ITIM). However, MHC Class I expression alone does not account for all NK inhibitory effects and other inhibitory receptors have been postulated. The applicant has recently identified a mouse MHC class I independent NK cell inhibitory receptor and its ligand, Nkrpld and Clrb. Unlike MHC class I molecules and MHC specific inhibitory NK cell receptors, these receptor and ligand gene families represented by Nkrpld and Clrb co-localize in the same genomic region, yet adjacent to MHC specific inhibitory NK cell receptors. Little is known in this novel MHC class I-independent NK cell regulation in vivo. Our longterm goal is to understand the mechanisms of NK cell regulation and tolerance, especially those that are MHC class I-independent. It is our opinion that the conceptual and genomic novelty of our findings and the proposed experimental plan represent an ideal fit for "Exploratory/Developmental Research Grant Award (R21)"(PA-03-107). The overall objective for this application is to understand the function and expression of Nkrpl and Clr family members. In particular, the specific aims are: 1) Elucidate the mechanisms of Clr expression. 2) Determine the role of Nkrpl/Clr genes in NK cell development. These studies are critical to our understanding of innate immunity and improving the manipulation of NK cells for therapeutic purposes.